This invention relates to electrical cardiac leads of the type used to transmit electrical signals from a pacemaker to the heart of a living subject, and in particular to improvements in such leads and their method of installation which allow more accurate installation with reduced damage to the tissue of the heart.
For some time, electrical leads have been used to conduct pacing signals from a pacemaker to the heart of a living subject. Such cardiac electrodes typically terminate in an electrode tip which must be surgically implanted in the heart to deliver the desired pacing signals to an appropriate portion of the heart. Screw-in electrodes can be implanted in the heart with relatively little trauma to the heart muscle. However, even with screw-in electrodes, accurate manual implantation can be difficult due to movement of the heart, and this movement can result in undesired laceration of the heart by the electrode before the electrode has become embedded in the heart.
Lacerations of the type described above can result in the formation of undesirable scar tissue near or immediately adjacent to the electrode. Such scar tissue tends to increase the voltage of the pacing signal required to pace the heart properly. This increased voltage in turn reduces the life of batteries used to power the associated pacemaker.
In addition, many commonly used screw-in electrodes use perforated, fabric skirts near the screw-in electrode as an aid in fastening the electrode to the heart. In use, these fabric skirts serve as bonding sites at which heart tissues can engage the electrode to hold the electrode firmly to the heart. Such bonding skirts present important disadvantages in that they do not significantly help to secure the electrode to the heart for some period of time after the electrode has been implanted in the heart. This is because heart tissues do not grow into and around the fabric skirt until a period of time ranging from several days to weeks has elapsed. During this period of time, prior to the attachment of the skirt to the heart, it is the screw-in electrode itself which holds the electrode in place.
Once fabric skirts of the type described above have become embedded in heart tissue, the electrode often cannot be removed from the heart without cutting the skirt and associated heart tissue from the heart. Thus, the removal of the electrode is a surgical procedure which serves further to traumatize the heart and can result in the formation of additional scar tissue.
Many conventional screw-in electrodes have electrodes approximately six millimeters in length. However, it has been established that the wall thickness of the muscle layer of the apex of the left ventricle of many human subjects is at its thinnest point less than two millimeters in thickness. Thus, there is a very real danger that screw-in electrodes of the conventional type may perforate the ventricular wall. See, for example, the discussion of this problem published in Vol. 39 of the British Heart Journal at pages 806-809 (1977).